The major goal of this project is to determine how the magnitude, pattern, and growth of hearing loss and structural damage are altered as the physical characteristics of the stimulus (frequency, intensity, duration, and scheduling of noise) are varied. Another goal is to determine the physiological correlates (as revealed by single-unit studies) of the behavioral changes and anatomical damage induced by exposure to noise. These behavioral, physiological and anatomical studies share the long-term goal of identifying the relations between functional deficits and injury in the inner ear rather than specifying damage-risk criteria for human exposures. In addition to these studies, acoustic measures (distortion products; spontaneous otoacoustic emissions) will be made from ear canals of noise-exposed chinchillas. These measures will provide physical indicators of cochlear integrity in the intact animal and information about nonlinear and active processes in the normal and damaged cochlea. Some of the experimental animals will be trained by food-reward, operant-conditioning techniques so that measures of auditory function (e.g. pure-tone thresholds, frequency DL's, etc.) can be obtained before, during, and after their exposure to noise. Physiological studies will include thresholds and tuning curves of individual fibers, rate-level functions and population studies in noise-exposed, behaviorally-trained animals. Histological evaluation of the specimens will include detailed study of plastic-embedded flat preparations so that counts of missing sensory and supporting cells can be made and cytocochleograms prepared for all specimens. Selected regions of the organ of Corti will be sectioned and examined by light and electron microscopy and analyzed morphometrically.